1. Field of the Invention
The invention relates generally to full duplex transceivers and more particularly to a full-duplex transceiver having a method for immunizing itself against self-jamming.
2. Description of the Prior Art
A transceiver can be designed as full duplex or half duplex. A full duplex transceiver receives and transmits signals simultaneously. In contrast, a half duplex transceiver receives signals during different time periods than it transmits signals. In a communication system, the simultaneous transmission and reception of signals results in a full duplex transceiver having a higher information rate than a half-duplex transceiver. However, a full duplex receiver is susceptible to a problem called “self-jamming”.
Self-jamming occurs when a portion of the power of the transmit signal crosses over into the receiver. Because the power level in the transmit signal is so much greater than the power level in the receive signal, even a small portion of the transmit signal power can overwhelm the receive signal and jam the receiver so that the receiver cannot properly receive the receive signal.
Existing full duplex transceivers attempt to resolve the self-jamming problem by transmitting and receiving signals in different frequency bands and then filtering the input to the receiver to pass signal frequencies in the receive frequency band while suppressing signal frequencies in the transmit frequency band. However, practical filters can never suppress 100% of the transmit signals and filters designed to approach 100% suppression are difficult and costly to produce. Moreover the transmit and receive frequency bands may be adjacent or nearly adjacent in order to meet licensing regulations, thereby further increasing the costliness and difficulty of producing the filters. Surface acoustic wave (SAW) filters are used in order to provide the very sharp transitions required for filtering adjacent or nearly adjacent frequency bands. However, SAW filters have a disadvantage that they cannot be integrated into an integrated circuit with standard integrated circuit technology.
Another approach for resolving the self-jamming problem involves the use of separate antennas for transmitting and receiving signals. However, this approach is not entirely successful because it increases cost, and the receive antenna inevitably receives some portion of the high power transmit signals through the air from the transmit antenna. Other approaches involve the use of directional devices, such as isolators, circulators, hybrid couplers and the like, for separating the transmit signals from the receive signals in a shared antenna. However, these approaches are only partially successful because limitations in the directivity of the directional devices and impedance mismatches in the directional device and antenna cause some portion of the transmit signal to cross over into the receiver. Typically, a combination of these approaches is used. However, none of the approaches or combinations tried to date has been entirely successful at a reasonable cost.
There continues to be a need for resolving the problem of self-jamming in a full duplex transceiver.